1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Brief Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a conventional lithographic projection apparatus, during photolithographic processes, an article, such as a wafer or reticle is clamped on an article support by a clamping force, that may range from vacuum pressure forces, electrostatic forces, intermolecular binding forces or just gravity force. The article support defines a plane, in the form of a plurality of protrusions defining an even flat surface on which the wafer or reticle is held. Tiny variations in the height of these protrusions are detrimental to image resolution, since a small deflection of the article from an ideal plane orientation may result in rotation of the wafer and a resulting overlay error due to this rotation. In addition, such height variations of the article support may result in height variation of the article that is supported thereby. During the lithographic process, such height variations may affect image resolution due to a limited focal distance of the projection system. Therefore, the article support may be flat.
European patent application EP0947884 describes a lithographic apparatus having a substrate holder wherein protrusions are arranged to improve the flatness of the substrate. These protrusions have a general diameter of 0.5 mm and are located generally at a distance of 3 mm away from each other and thereby form a bed of supporting members that support the substrate. For an electrostatic clamp, typically, the height of the protrusions lies in the range 1 mu m–15 mu m. Due to the relative large spaces in between the protrusions, contaminations possibly present generally do not form an obstruction for the flatness of the substrate, since these will be lying in between the protrusions and will not lift the substrate locally.
In the context of this application, the “article” may be any of the above mentioned terms wafer, reticle, mask, or substrate, more specifically terms such as: a substrate to be processed in manufacturing devices employing lithographic projection techniques; or a lithographic projection mask or mask blank in a lithographic projection apparatus, a mask handling apparatus such as mask inspection or cleaning apparatus, or a mask manufacturing apparatus or any other article or optical element that is clamped in the light path of the radiation system.
In lithographic processing, passing of the projection beam through gas compositions present between the illumination system and the articles to be illuminated, in particular non-homogenous gas compositions, may cause undesired effects such as diffraction, refraction and absorption. These effects may have an adverse effect on illumination quality, in particular on a suitable resolution to be reached for the ever increasing demand in imaging performance. A new generation of lithography, the EUV-lithography, which uses a projection beam in the Extreme Ultra Violet area, therefore operates in or near vacuum conditions in order to allow the projection beam of radiation to pass substantially unhindered to the article to be placed in the beam. In this context, the term vacuum pressure is relative to particular gasses that are present in the environment. For instance, for carbonhydrogens and water, the allowable background pressure is very low, in the order of 1e−9–1e−12 millibar. For inert gasses the requirements are less strict, for instance, for Ar an allowable background pressure ranges from 1e−4 mbar–1e−2 mbar, in particular, a pressure of 1e−3 mbar. Also, the relative background pressure may vary in terms of the environment of the apparatus. For instance, where the article support functions in the environment of a wafer support, the vacuum requirements for certain components may be less strict than in the environment where the article support functions as a reticle support. That is, the partial pressures for contaminants (such as CxHy and H2O) may differ a factor 100 between optics compartment (including reticle support) and wafer compartment and are much lower than the total pressure (typical numbers are 1e−9 to 1e−12 mbar).
This vacuum technology offers challenges in terms of temperature control. For the article support of the preamble, only a very small part (ranging from 0.1 to 3% of a total area) of the bottom side of the article actually makes physical contact with the article support when supported thereby, since the protrusions are shaped to provide only a very small contact area and the protrusions are furthermore arranged spaced relatively wide apart. In the vacuum pressure ranges that are used, thermal conductivity is substantially proportional to the pressure, which means that the thermal energy absorbed by the article when placed in the projection beam can no longer adequately be diverted, so that unwanted thermal heating of the article supports, leading to thermal expansion and resulting projection inaccuracies or potentially to even the loss of the article. To overcome this problem, usually, use is made of a so-called back-fill gas, which offers a thermal conduction from the article to the article support to divert the thermal energy absorbed by the article. Of course, the article support will further, if necessary, be provided with a cooling structure such as cooling ducts containing cooling media etc. However, to confine the back-fill gas to the bottom side of the article, a conventional approach is providing a so-called “hard rim”, which is a boundary wall substantially sealing of the backfill gas from the vacuum by forming a gas seal between the bottom side of the article and the upper side of the article support. A hard rim type article support is for instance known from European patent serial number EP1241706.
It has been found, however, that, in terms of illumination performance, such a hard rim causes problems. For an article such as a substrate at wafer level or a reticle in a reticle stage, the rim may result in an estimated deflection of 75 nm, causing a deteriorated image resolution. The presence of a sealing rim provides additional support to carry the article. Such additional support disturbs the pressure load of the article, which may cause local bending of the article. Such bending introduces rotation of the article surface, which may cause overlay effects that are undesired. Furthermore, a sealing rim provides almost a doubling of increase of contact area between the article and article support. This is undesirable since it is an aim to minimize such contact area in order to prevent contamination particles to come in between the contact zones, creating unevenness of support and corresponding bending problems of the article.
Furthermore, the presence of such a hard rim defines a definite outer area of the article where no backfill gas is present to provide thermal conductivity. This may cause further problems in terms of local overheating or undesired temperature gradients in the article.